Material can be removed selectively from a substrate surface by masking the surface areas from which no material shall be removed and exposing the remaining, free areas to an etchant. Based on the non-masked areas, material can then be removed depthwise from the substrate. However, this may involve various problems:                The etchant does not only etch the exposed surface areas but also the masking material. Depending on the duration of exposure, the masking can be thinned or fully be removed, followed by a removal of material from the substrate surface which should actually be protected.        Underetching may result, i.e. etching occurs from the side walls of the already produced deepening laterally under the mask so that the edges under the masking layer have a fringed and non-defined appearance. Moreover, the walls of the resulting deepening are not smooth.        Non-uniform etch rates within a single deepening and/or over several deepenings on a substrate result in undefined depths.        Undesired redepositions of material removed by etching. Substrate and/or masking material removed by etching can deposit in an undesired way or at unfavorable sites on the substrate and/or the etching device and lead to unusable results or no longer operating etching devices.        When deep deepenings (depth T>200 μm) are produced, the etch rate can be too low to yield economically efficient results.        
Etch processes may be isotropic (i.e. have equal effects in all directions) or anisotropic (i.e. have effects better in some directions in space than in other directions in space). Wet etching is a usually isotropic etching process which is, however, relatively slow and cannot be used for etching deep deepenings, e.g. in a silicon wafer. Dry etching has a higher etch rate (removal per unit time). In this case, an etch plasma (SF6, for example) is produced and placed on the sites to be etched for the purpose of exposure. In this connection, the term “plasma” is also understood to mean highly ionized (not fully ionized) states of matter. This is also referred to as RIE (reactive ion etching). In so far as deep deepenings have to be produced, this is referred to as DRIE (deep reactive ion etching) if dry etching is employed. Here, the special demands are made on the homogeneity of the etch process and the resistance of the masking material. Another increase in the etch rate can be achieved by means of ICP etching. In this method, highly ionized plasma is produced by inductive power coupling (ICP=inductively coupled plasma). The etch rates are so high that using conventional masking layers of polymers or oxides only poor deepening depths can be obtained before the masking layer is removed by etching along with the substrate.
On the other hand, it is known to use masking layers containing or being fully made (>98% by weight) of metallic materials, in particular aluminum. They have the property that even in the case of thin masking layers they have, also in the case of ICP etching, such a resistance that deep deepenings can be produced before the masking layer has been removed. Nevertheless material is also removed by etching from the masking layer. It accumulates inter alia in the etching device where it is found in or on the tubes supporting the inductive coupling. As a result, the tubes become metallically conductive so that the inductive coupling and thus the etch rate initially deteriorate and ultimately collapse. This leads to an expensive and time-consuming cleaning of the device.
The underetching of the masking layer is avoided by a method as known from U.S. Pat. No. 5,501,893. In short, this methods alternately (with a periodicity of some few seconds) supplies etch gas and a passivation gas to the surface to be etched. With a suitable layout, the passivating agent in the passivation gas deposits on the side walls of the deepening so that the etch gas only etches the bottom of the deepening thus avoiding underetching and producing approximately perpendicular walls.
It is the object of this invention to provide an etching method which permits the formation of deep deepenings at a high etch rate.
This object is achieved by the features of the independent claims. Dependent claims are directed to preferred embodiments of the invention.